The game is set in the fictional world of Sanctuary, centred around a group of anonymous heroes who are trying to catch up and stop the Prime Evil known as Diablo before he frees his brothers and destroys the mortal realm.

Diablo II was one of the most popular games of 2000. Major factors that contributed to Diablo II's success include its addictive gameplay and access to the free online play service, Battle.net.

An official expansion pack, entitled Diablo II: Lord of Destruction also developed by Blizzard North, was released on 29 June 2001. A sequel called Diablo III was announced to be in production on June 28, 2008 at WWI 2008 in Paris, but will be developed by Blizzard Entertainment, since Blizzard North has dissolved.

Diablo II is built around Acts that progresses the narrative story line of the game, with each act following a predetermined path and list of quests, with small selection of optional quests.

The player assumes the role of a hero from one of five different character classes, fighting monsters through wilderness areas and dungeons in order to level-up their character and gain better items. The game uses real-time combat and the player sees the character from an isometric viewpoint. Players also have the option of hiring one NPChirelings that accompanies the player and attack nearby enemies. A powerful boss monster awaits the player at the end of each Act. Item drops, monster attributes, and most dungeon layouts are randomly-generated by Diablo II. The game also continues the use of randomised affixes from Diablo I.

After completing the four Acts in the game, there are two additional sequential difficulty levels, letting the player go through all three before reaching the max level at 99. These are: Normal, Nightmare, and Hell. On higher difficulties, monsters are stronger, experience is penalized on dying, and the player's resistances are handicapped. A character retains all abilities and items between difficulties, and may return to a lower difficulty at any time through a waypoint system.

Players can also create a hardcore character, as opposed to a normal (popularly called "softcore") character. A normal character can be resurrected if killed and resume playing, while a hardcore character has only one life. If killed, the character is permanently dead and unplayable, and all items and equipment on that character will be lost.

Diablo II allows the player to choose between five different character classes: Amazon, Necromancer, Barbarian, Sorceress, and Paladin. Each character has different strengths and weaknesses and sets of skills to choose from, as well as varying beginning attributes.

The Amazon hails from the islands of the Twin Seas, near the border of the Great Ocean. The class is based on Greek mythology Amazons. She is similar to the Rogue of Diablo I as they use ranged weaponry and use both strength and magic. The Amazon also uses javelins and spears. Many of her passive skills are defensive in nature, especially Dodge, Avoid, and Evade. The Amazon is voiced by Jessica Straus.

The Barbarian is a powerful melee fighter from the steppes of Mount Arreat. He is an expert at frontline combat, able to absorb great punishment like a "tank", and is the only class capable of dual-wielding weapons. His skills allow him to specialise in different types of weapons, increasing his resistances, speed, and defence. He can also use ancient Barbarian Warcries and combat techniques to buff himself and others as well as dealing with any foes quickly and violently. The Barbarian is voiced by David A. Thomas.

The Sorceress hails from a rebellious mage clan of female-only Sorceresses who have wrested the secrets of magic use from the male-dominated clans. She wields the elements of ice, lightning and fire spells, using mostly offensive spells. Her Cold Skills can freeze enemies solid and bypass resistances, but do less damage than lightning or fire. The Sorceress's Teleport spell allows her to instantly travel to a new destination, making her very difficult to hit. As such she is a "glass cannon", dealing much damage, but can't take much in turn. The Sorceress is voiced by Liana Young.

The Paladin is a crusader from the Church of Zakarum, fighting for the glory of the Light. The Paladin's combat skills range from fanatical attacks to heavenly thunderbolts and blessed auras. The auras can incre damage, or resist magic attacks, or boosting defence. The Paladin's auras will buff all party members, making him very useful for multiplayer. The shield is another Paladin speciality, and he is the only character that can use it as a weapon. The Paladin also have very powerful anti-undead skills. The Paladin is voiced by Larry B. Scott.

The monsters of Diablo II are more numerous than the ones in Diablo I, and has slightly more abilities of their own, but is otherwise quite similar in style of gameplay. Some monsters have received improved AI, like the Fallen monsters, who attack slightly more coordinated, and ranged monsters who retreat if the player moves up to melee range. A few boss encounters also use more "intelligent" combat, like special abilities from the boss, rather than the fight just being a race to drink potions.

The potions work similarly to the original. It's gameplay purpose is basically that of an extended health/mana bar, with no cooldowns on usage, and little delay in filling up the resource pools. The big change is how the belt works, where instead of 1-0 selected slots, the player now only has 4 slots, but as he levels up, he can get better belts, which can save more rows of 4 potions, making the actual maximum of stored potions up to 16. Scrolls can not be stored in the Diablo II belt, but specific "tome" items that takes up 2 slots can be purchased to save space, storing up to 20 scrolls in each.

Items use a very similar mechanic to the original game, with affixes on magical items, an uniques that are more powerful. On top of that, set items and rares were introduced. Basically better versions of magical items, but not as special as uniques.

Diablo II can be played multiplayer over or Blizzard's free online service Battle.net. Unlike the original Diablo I, Diablo II was made specifically with online gaming in mind. Several spells (such as auras or Warcries) multiply their effectiveness if they are cast within a party and the game world included a lot more open spaces.

The Diablo II Battle.net is divided into "Open" and "Closed" realms. Players may play their single-player characters on open realms while characters in closed realms are stored on the servers as a measure against hacking and cheating. The character must also be played every 90 days to avoid expiration. Online play is otherwise nearly identical to single-player play. The most notable difference is that online maps are generated randomly every time a new game is entered, while offline single player maps are just generated when the game is first started as a persistent world the player will always return to when playing with that character.

The game is most often played cooperatively (PvM), and the right class combination with complementary skills can finish some of the game's climactic battles in a matter of seconds, providing strong incentives for party-oriented character builds. Up to eight players can be in one game but are not automatically made into a party. It's also possible to form multiple opposing parties. Experience gained, monsters' hit points and damage, and the number of items dropped are all increased as more players join a game, though not in a strictly proportional manner. Players are allowed to duel each other with all damage being reduced in PvP. The bounty for a successful kill in PvP is a portion of the gold and the "ear" of the defeated player (with the previous owner's name and level at the time of the kill).

Due to the PvP mechanics, the game has a lot of non-consensual PvP combat, also called "Player Killing", PK.

Patch 1.10 included the option of playing with a ladder character. The ladder system can be reset at various intervals to allow for all players to start fresh with new characters on an equal footing. Ladder seasons have lasted from as short as nine months to over a year. When a ladder season ends all ladder characters are transferred to the non-ladder population. Certain rare items are available only within ladder games, although they can be traded for and exchanged on non-ladder after the season has ended.

The game has been patched extensively; the precise number of patches is impossible to determine as Battle.net has the capability of making minor server-side patches to address immediate issues. The game is currently in version 1.12 with the latest major patch released on June 17, 2008. Through the patch history, several exploits and issues have been addressed (such as illegal item duplication, though it still exists), as well as major revamps to the game's balance. Not all patches have affected Diablo II directly, as several were designed to address issues in the expansion to the game and had minimal effects on Diablo II.

On March 3, 2009, Blizzard announced a new Diablo 2 content patch, 9 years after the game's release. Diablo 2 content patch 1.13 has been released on a public test server named "classic beta".

The story of Diablo II takes immediately after the end of Diablo (and also in Sanctuary), where the Warrior character of the first game tried to contain Diablo within himself by plunging the Soulstone into his own head. Unfortunately, he was instead corrupted by the Lord of Terror, and became the Dark Wanderer. Despite struggling with the demon within, he spread chaos and terror wherever he went. Diablo had in fact planned this all along, luring the Warrior to his dungeon.

The player takes on the role as one of five (seven in the expansion) heroes who group up in order to stop Diablo and the other Prime Evils. They start in the Rogue Encampment (which may be in Entsteig or Khanduras, the lore is fuzzy), chasing after this mysterious Dark Wanderer who had summoned demons to overrun their monastery, which guarded the passage to the East.

Diablo leaves hinders for the heroes, such as the Lesser Evil, Andariel, in the Monastery, and heads first to the Aranoch desert to free his brother Baal near Lut Gholein, and then further to the jungles of Kehjistan to free Mephisto under the city of Kurast. At every step of the way, the player not only chases Diablo, but also has to deal with demonic corruption and other obstacles in the way.

Finally, the heroes catch up with the Brothers, and kill Mephisto in Travincal, ending the corruption of the Zakarum Church, and goes through a portal to Hell itself, to find Diablo.

Searching hell, and finding the Chaos Sanctuary, Diablo's home, the heroes summon the demon before he has fully recovered from transforming into his real, red, self, and defeats him. The heroes are then charged with destroying the Soulstones as the Prime Evils were taught long ago how to corrupt them by the fallen angelIzual.

The story continues in the expansion to the game, where the player chases the last of the Prime Evils; Baal (Lord of Destruction) who is going after the mythical Worldstone in an attempt to corrupt it.

The "Secret Cow Level" is the result of a running joke from the original Diablo that spawned from an Internet rumor about the cow placed in the town of Tristram seemingly without purpose. Supposedly, if the cow was clicked on a certain number of times, a portal to a secret level would open. The rumour was a hoax, but the legend was born as player after player asked Blizzard about how to access the level, and they vehemently denied its existence.

In the Diablo I expansion Diablo: Hellfire, created by third-party developer Synergistic Software, it was possible to change a parameter in a specific text file, so that the farmer was dressed in a cow suit, with appropriate new dialogue ("Moo." "I said Moo!"). This added fuel to the fire. To stop the rumours, Blizzard included a cheat in StarCraft that read "There is no cow level", officially confirming that there was, in fact, no cow level.

On April 1, 1999, a Diablo II Screenshot of the Week featured cows fighting. People wondered if the screenshot was an April Fool's joke or if there really was a Secret Cow Level planned for Diablo II. It turned out that there was a cow level in Diablo II.

The game was also released in Collector's Edition format, containing bonus collector's material, a copy of the Diablo Dungeons & Dragons pen-and-paper campaign setting, and promotional movies for other Blizzard games. The Diablo II: Exclusive Gift Set (2000) similarly contained exclusive collector's material and promotional videos, as well as a copy of the official strategy guide. The Diablo Gift Pack (2000) contained copies of Diablo I and Diablo II, but no expansions. The Diablo: Battle Chest (2001) contained copies of Diablo II, Diablo II: Lord of Destruction, the official strategy guide, and the original Diablo. Recently however the Battle Chest no longer contains the original Diablo.

Unlike Diablo I, Diablo II did not come with a "spawn" version, and was not released on PlayStation, as the team felt the gameplay was not optimal for console play. The game did, however spawn an expansion: Diablo II: Lord of Destruction, which was widely positively received by the fans of the franchise.

The announcement of Diablo III has renewed the interest in its predecessor and brought more attention to the many mods available for the game.

The score has been composed by Matt Uelmen and integrates creepy ambience with melodic pieces. The style of the score is ambient industrial and experimental. Some tracks were created by reusing the tracks from the original game, while others by rearranging tracks that were out-takes. Other scores are combinations of parts that were created more than a year after the first game's release.

While the player visits a town, the game has to create a much more peaceful atmosphere, so for that the town theme from Act I called Wilderness gives a pastoral feel of the wilderness (with the cows, farm fences, cabins and trees).

For Act II Mustafa Waiz, a percussionist, and Scott Petersen, the game's sound designer, worked on the drum samples. Mustafa played on the dumbek, djembe, and finger cymbals which gave Matt Uelmen a base upon which to build tracks around.

The town theme from Act II, called Toru makes strong statement of departure from the world of Act I while also maintaining a thematic connection to what had come before. It is the first time in the series to be used some radically different elements than the guitars and choral sounds that dominate both the original Diablo and the opening quarter of Diablo II.

The foundation of the Toru piece is found in exciting dynamics of a Chinese wind gong. The instrument radically changes color from a steady mysterious drone to a harsh, fearsome noise, that gives exotic feeling and at the same time the pacing of the second town. In all sequences of Act II with deserts and valleys, Arabic percussion sounds dominate.

The composer was impressed by two of the Spectrasonics music libraries, Symphony of Voices and Heart of Asia. He uses samples from Heart of Asia in the Harem piece from Act II, and tries to put the sampled female voice out front and center, getting a nice alto in it. The Crypt track uses a sample from Symphony of Voices; the choral phrase Miserere. deep in the mix of the track, alongside the excessive rainsticks and cymbal scrapes, combined with metal guitar solos.

Diablo II had a positive reception. The game has achieved an overall score of 88 on Metacritic. Gamespy awarded the game an 86 out of 100, IGN awarded the game an 8.3 out of 10, and Gamespot awarded the game an 8.5 out of 10 along with earning the 2000 runner-up Reader's Choice Award for role-playing game of the year. It was awarded a spot in the Guinness Book of World Records 2000 edition for being the fastest selling computer game ever sold, with more than 1 million units sold in the first two weeks of availability.

As of August 29, 2001, Diablo II has sold 4 million copies worldwide. The game has received the "Computer Game of the Year", "Computer Role Playing Game of the Year", and "Game of the Year" awards from the Academy of Interactive Arts and Sciences at the 2001 Interactive Achievement Awards.